Similar to sapphire (Al2O3), magnesium oxide (MgO) is a typical oxide insulator having a wide gap. Therefore, MgO is sometimes used as a substrate to grow a thin film. When MgO is used as a growth substrate, matching of a lattice constant of the MgO substrate with respect to a lattice constant of a growing thin film is considered. As a crystal face of MgO on which a thin film is grown, for example, a plane whose plane orientation is (100) is used in some cases (refer to, for example, Non-Patent Document 1). Non-Patent Document 1 describes Fe3O4(100)/MgO(100)/Fe3O4(100) as a configuration that provides a TMR effect. MgO and Fe3O4 have a lattice mismatch of 1% or less therebetween, which suggests that MgO is suitable as a growth substrate for Fe3O4.
On the other hand, as a crystal face of MgO on which a thin film is grown, for example, a plane whose plane orientation is (111) is used in some cases (refer to, for example, Non-Patent Documents 2 and 3). Non-Patent Documents 2 describes that the coercivity and residual magnetism are increased by forming a ferromagnet having a spinel structure on a MgO(111) plane. Non-Patent Document 3 describes forming a (Ba,Sr)TiO3 thin film, which is a typical ferroelectric thin film, on a (111) plane or on a (100) plane of a MgO substrate. Non-Patent Document 3 further describes that forming a (Ba,Sr)TiO3 thin film on the (111) plane of a MgO substrate results in satisfactory characteristics in a high-frequency range as compared with when forming the same on the (100) plane.
As described above, the (100) plane and (111) plane of MgO can be applied to a variety of films as growth substrates. Also, the (111) plane of MgO provides advantageous effects as compared with those of the (100) plane in some cases. In addition, Non-Patent Document 4 describes that a (111) plane of MgO that results in a root mean square roughness RRMS of 0.21 to 0.23 nm can be obtained by forming NiO(111) on yttria-stabilized zirconia (YSZ) having a (111) plane as its principal surface and growing MgO on said NiO(111). Here, the root mean square roughness RRMS is also represented by Rq (quadratic mean roughness) (JIS B0601).